Cardiovascular system Flashcards
Describe the structure of the heart.
- The walls of the heart = myocardium
- Right and left halves both have an atrium and ventricle
- Between the atrium and ventricle are the atrioventricular valves: tricuspid on the right and bicuspid on the left. These permit blood to flow from the atrium to the ventricle but not vice versa.
- Semi-lunar valves control the passage of blood between the ventricles and the main arteries (aorta and pulmonary artery).
- Blood leaves the right ventricle through the pulmonary artery and passes through the lungs. It collects oxygen and returns to the left atrium of the heart through the pulmonary vein. Blood passes into the left ventricle and out through the aorta around the rest of the body. It returns to the right atrium via the vena cava.
Describe the coordination of the heartbeat.
- The initial depolarisation arises in a small group of cells called the sinoatrial node in the right atrium, near the entrance of the vena cava. The SAN is the pacemaker of the heart.
- The depolarisation spreads through the muscle cells of the right atrium by way of gap junctions, causing the atrium to contract.
- The action potential passes on to the atrioventricular node at the base of the right atrium. The propagation of action potential though the AV node is relatively slow allowing atrial contraction to be completed before ventricular excitation occurs.
- The action potential then propagates down the wall between the two ventricles (the interventricular septum) via the bundle of His.
- The bundle of His divides into right and left branches which enter the walls of both ventricles and make contact with Purkinje fibres which distribute the impulse through the ventricles. Depolarisation and contraction begin slightly earlier at the bottom and rise to the top which acts to squeeze the blood out towards the excite valves.
- Once contraction has completed a period of repolarisation occurs.
Describe how the heart rate can be controlled.
The SA node has a basic rhythm but this can be increased or decreased by activity by activity in sympathetic and parasympathetic nerves fibres innervating the node. Two nerves link the cardiovascular centre in the medulla oblongata of the brain with the SA node of the heart:
- Accelerator nerve (sympathetic NS) - releases neurotransmitter when stimulated at the SA node to increase heart rate.
- Vagus nerve (parasympathetic NS) - when stimulated releases neurotransmitter at the SA node to decrease heart rate
Parasympathetic activity slows the rate at which depolarisation reaches the threshold e.g. acetylcholine opens potassium channels so potassium will flow down its concentration gradient out of the cell so the cell will take longer to reach its threshold.
Sympathetic activity speeds up the rate at which depolarisation reaches its threshold e.g. Adrenaline opens sodium and calcium ion channels so positive ions enter into the cell more quickly hence it depolarises faster.
Describe the cardiac action potential
- Na ion entry depolarises the cell and leads to more sodium channels opening in a positive feedback manner
- The permeability to K+ decreases contributing to the membrane depolarisation.
- The membrane remains depolarised for a plateau period as the potassium channels remain closed and the depolarisation opens voltage gated calcium channels. Ca2+ flows down its electrochemical gradients into the cell which balances the flow of positive potassium ions out of the cell so keeps the membrane depolarised.
- Depolarisation ultimately occurs when calcium channels inactivate and potassium channels open
Explain the shape of the ECG.
The electrocardiogram is a measure of the electrical currents in the extracellular fluid by changes occurring in the heart (no measure of mechanical activity).
- The first deflection (P wave) corresponds to current flows during atrial depolarisation
- The second deflection (QRS complex), occurring approximately 0.15s later, corresponds to current flows during ventricular depolarisation. This is complex because the wave of depolarisation through the ventricular walls differ from instant to instant and the currents generated in the body fluids change direction accordingly.
- The final deflection (T wave) is the result of ventricular repolarization.
- Atrial depolarisation is not usually evident on the eCG because it occurs at the same time as the QRS complex.
Describe the cardiac cycle
- Diastole (Relaxation of the heart)
- Atria and ventricles are relaxed
- Blood returns to the atria
- Pressure rises in the atria and when it exceeds that in the ventricles the atrioventricular valves open so blood passes into the ventricle
- Relaxation of the ventricle walls causes the pressure in the ventricles to be lower than in the aorta and pulmonary artery so the semi-lunar valves close. - Atrial systole (contraction of the atria)
- Atria contracts, ventricles stay relaxed
- Contraction of atrial walls pushes the remaining blood into the ventricles - Ventricular systole (contraction of the ventricles)
- Atria relax, ventricles contact
- Increased pressure in the ventricles force the AV valves shut
- Once pressure exceeds that in the aorta and the pulmonary artery the semi lunar valves open and blood is forced into these arteries.
What is cardiac output?
- Amount of blood ejected from the heart per minute
CO = HR x SV
Stroke volume = volume of blood ejected from each ventricle during systole
How can the stroke volume be controlled?
- End diastolic volume:
- how much the ventricles are filled. This controls the amount of blood in the ventricle available for ejection.
- Sympathetic regulation:
- Degree of ventricular contraction (mainly controlled by the level of circulating adrenaline).
Describe the sympathetic regulation of heart contraction.
- Adrenaline binds to a G-protein coupled receptor.
- This causes the activation of adenylate cyclase thus conversion of ATP to cAMP.
- cAMP activates Protein Kinase A which phosphorylate proteins involved in excitation of the heart
- These cause voltage-gated calcium channels to open increasing the influx of Calcium and enhancing contractility.
How does exercise affect cardiac output?
- energy demands of exercise require increase blood flow to active tissues increasing SV and increasing HR.
Describe the structure of an artery.
- Thick muscular layer - control blood flow
- Thick elastic wall - maintain pressure
- Thick wall - prevent bursting under pressure
- No valves as constant high pressure
Describe the structure of arterioles.
- Thick layer of smooth muscle to allow it to vasodilate (increase in diameter) or vasoconstriction, so they can direct blood flow into capillary bed.
Describe the structure of capillaries
- Once cell thick endothelium - thin wall reduces diffusion distance.
- Slow movement of blood through capillaries maximises the time for substances to exchange across the capillary wall.
- Many and highly branched for large SA for exchange
- Precapillary sphincters can close off capillary beds.
Describe the structure of a vein
- Thin muscle layer
- Thin elastic layer as low pressure
- Valves at intervals to stop blood flowing backwards.
- Walls largely made up of collagen fibres making them very distensible and able to accommodate an increase in blood volume without much rise in pressure
How is blood flow controlled?
- Regulation of the diameter of blood vessels.
- Arterioles are the major site of resistance to flow. Smooth muscle of their wall can regulate the diameter of the blood vessel by:
- contracting (vasoconstriction) or
- relaxing (vasodilation)
